[TowerTalk] Latteral load capability of T2X in mast mounted

[Jim Thomson]

Date: Sat, 22 Jul 2017 00:46:45 +0800
From: "Jeff AC0C" <keepwalking188 at ac0c.com>
To: <towertalk at contesting.com>
Subject: [TowerTalk] Latteral load capability of T2X in mast mounted
configuration

<I am using a mobile crank-up / tilt tower for some antenna testing that has a top section too small to get a ham4 rotor in.  Unfortunately that means I need to mount the rotor on a pole that extends from the top of the tower.  

<The good thing about this setup is the tower nests and the tilts down essentially parallel to the ground, allowing you to mount the antenna while standing on the ground.  The bad thing about that setup is the rotor has got to be stout enough to hold the antenna weight while the rotor is temporarily horizontal.  

I have used a ham4 in the past but that was with antennas which were pretty light weight (under 75 lbs).  This time the antennas will be somewhat more heavy ? up to about 200 lbs ? and I have a tailtwister available.  

Would the tailtwister would be happy holding that much weight off the nose when it?s temporarily horizontal?

Higain says the load capacity for a mast mounted configuration is half of a inside-tower-mount case but I have no idea if that is applicable in the case of the rotor being mounted horizontally.  Seems like it would because the rotor is going to feel a lot of torque in the externally mounted case with serious wind.  

In my case wind is not an issue.  I don?t leave the antenna on the tower when I?m not testing it.  So the only question is how the tailtwister would do with a couple hundred pounds pulling down on the top when the tower is tilted over?

Appreciate any comments or insights.  

73/jeff/ac0c

###  With 200 lbs much above the T2x, the bending moment on the rotor assy  will be massive.   Think of it as a big TQ wrench.  2 ft X 200 lbs = 400 ft labs.  Do you really require the rotor  for the testing application ?   What might work easier, 
is to just use 1-2 bearings in the top of the tower,  with a short piece of mast above the tower.    Then the mast is free to rotate 360 degs.   Mount the yagi onto the mast, as close to the top of the tower as you can.  Then with some rope onto
the boom, say at the end of the boom, or towards the  end of the boom.   Then in the upright position, you will be able to orient the yagi in whatever direction you want.  Then raise the tower to desired height, and increase slack on rope of course. 
We did just this at several past field day events, on both crank up, and also guyed towers.   Walk with rope in hand to desired yagi azimuth heading, then tie down the rope.    No need for a rotor  for a temp set up. 

##  The real trick will be.... with the tower in horz position, mounting the yagi onto the mast.   The eles of the yagi cant be 90 degs to the mast,  or the tips will be embedded into the dirt.  Eles will have to be inline with the tower.   If the els are really long, like a 30M yagi, then you also have to worry about the nested height of the tower  vs  one half of the 30M ele length.   Once tower is moved to the vert position, somebody is gonna have to go up there, then rotate the boom clamp assy, by 90 degs so the eles are parallel to the dirt. 

Jim   VE7RF